netdev - Re: [PATCH 3/6] sched,perf,kvm: Fix preemption condition

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Message-ID: <20210602143016.GE12753@C02TD0UTHF1T.local>
Date:   Wed, 2 Jun 2021 15:30:16 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
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        Borislav Petkov <bp@...en8.de>, x86 <x86@...nel.org>,
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Subject: Re: [PATCH 3/6] sched,perf,kvm: Fix preemption condition

On Wed, Jun 02, 2021 at 04:10:29PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 02, 2021 at 09:59:07AM -0400, Mathieu Desnoyers wrote:
> > ----- On Jun 2, 2021, at 9:12 AM, Peter Zijlstra peterz@...radead.org wrote:
> > 
> > > When ran from the sched-out path (preempt_notifier or perf_event),
> > > p->state is irrelevant to determine preemption. You can get preempted
> > > with !task_is_running() just fine.
> > > 
> > > The right indicator for preemption is if the task is still on the
> > > runqueue in the sched-out path.
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > > kernel/events/core.c |    7 +++----
> > > virt/kvm/kvm_main.c  |    2 +-
> > > 2 files changed, 4 insertions(+), 5 deletions(-)
> > > 
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -8568,13 +8568,12 @@ static void perf_event_switch(struct tas
> > > 		},
> > > 	};
> > > 
> > > -	if (!sched_in && task->state == TASK_RUNNING)
> > > +	if (!sched_in && current->on_rq) {
> > 
> > This changes from checking task->state to current->on_rq, but this change
> > from "task" to "current" is not described in the commit message, which is odd.
> > 
> > Are we really sure that task == current here ?
> 
> Yeah, @task == @prev == current at this point, but yes, not sure why I
> changed that... lemme change that back to task.

FWIW, with that:

Acked-by: Mark Rutland <mark.rutland@....com>

I have no strong feelings either way w.r.t. the whitespace cleanup. ;)

Thanks,
Mark